A number of criteria are reviewed when determining effectiveness of a communication system including: cost, channel bandwidth, required transmitter power, signal-to-noise ratios, probability of bit error, time delay, and other criteria known in the art. In order to satisfy the above criteria various modulation schemes and coding methods have been developed.
In order to increase bit rate modulation constellations of more than two points, such as quadrature amplitude modulation (QAM) and phase shift keying (PSK), have been used at the cost of smaller Euclidean distances, distances between adjacent points in a signal constellation. The smaller the distance between the points the more difficult to decipher between adjacent points.
Additionally, coding is used to minimize errors in a received communication signal. Errors develop through transmission due to communication system and environmental effects on the communication signal. For example, a binary “1” may be converted to a binary “0” or vice versa in a transmitted communication signal.
One such common coding scheme is channel coding, which introduces controlled redundancy in order to reduce channel error rates. As redundant bits are added for coding purposes overall symbol rate increases for a particular information data rate causing bandwidth to increase. Another coding scheme, the so-called Trellis-Coded Modulation (TCM), combines modulation and coding to achieve coding gain without increasing bandwidth. Bandwidth efficient trellis-coded modulation schemes are employed to ensure performance of various communication channels including satellite channels for higher throughput.
Traditionally, two-dimensional (2-D) TCM employs 2m+1 symbols to transmit an information signal containing m information bits per symbol. Each bit corresponding to a possible “0” or “1”. Through coding m+1 coded bits are used to transmit m information bits. There are 2m+1 possible combinations of zeros and ones per symbol. Thus, the number of information bits m per transmitted symbol is an integer. For example, when transmitting four symbols per communication signal having two information bits per symbol, 12 coded bits are required, three coded bits per symbol. So when a communication system is required to send an information signal containing 9 information bits a full additional symbol must be used. The downfall to adding additional symbols is that the time of the completed transmission increases. If the time of the completed transmission is fixed the communication system power and bandwidth requirements will need to be increased to transmit one extra information bit. Therefore, the communication system is overbuilt and under utilized due to additional unused information bits. The additional requirements result in an inefficient and cost ineffective communication system.
Unfortunately with traditional TCM schemes, when m increases, coding gain increases more slowly and the error coefficient of the code begins to dominate performance. As the number of information bits is increased per symbol, constellations become difficult to create in 2-D. Additionally, cost of utilizing coded 2-D schemes is high, as compared to uncoded schemes, due to added redundant bits.
Multi-dimensional TCM provides higher coding gain and improved performance over 2-D TCM. Multi-dimensional TCM is used to reduce the number of redundant bits and constellation sizes and therefore reduce the manufacturing and operating costs. Several multi-dimensional schemes have been suggested, each having a large amount of constellation points in order to transmit a small number of information bits per symbol. The design purpose of the multi-dimensional schemes is to use additional dimensions over 2-D schemes to reduce the number of constellation points. However, it has been determined that the multi-dimensional schemes, although not designed to do so, may be used to transmit a fractional number of information bits per symbol.
Transmitting a fractional number of bits per symbol provides an appropriate amount of power and bandwidth for a desired amount of transmitted information bits and corresponding symbols and improves error performance. In other words, continuing from the above example the communication system may transmit 2.25 information bits per symbol on average instead of transmitting an additional symbol. The 2-D TCM fractional number of bits per symbol scheme has been suggested for 20-QAM, 24-QAM, 64-QAM, 96-QAM, and 112-QAM constellations. The 2-D TCM fractional number of bits per symbol scheme uses a partition tree to breakdown an initial constellation, at a top level, into multiple subsets, each subset having multiple representative constellations. A certain percentage of constellations in the lowest level subset have a first amount of uncoded bits and the remaining percentage have a second amount of uncoded bits. During modulation coded bits equally select between the lowest level subset constellations. Thus, in transmission a fractional average number of bits per symbol can be calculated depending upon the stated percentages.
Since the original design purpose of traditional multi-dimensional TCM methods was not to modulate a fractional number of bits per symbol, these methods are limited in effectiveness.
It would therefore be desirable to design a communication system transmitter and receiver that improves upon the above listed criteria including minimizing bit error rate, system complexity, and power consumption and is designed for the purpose of performing TCM for a fractional number of bits per symbol.
The goal in designing of a communication system is to minimize costs, channel bandwidth, required transmitter power, probability of bit error, time delay.